All-dielectric self-supporting cable having high fiber count

ABSTRACT

An all-dielectric self-supporting optical fiber cable utilizes a single layer reverse oscillated lay (ROL) design and includes a fiber count of more than 288 fibers. By arranging buffer tubes in a single layer, the ADSS cable effectively isolates the tensile and thermo strain of the cable in central and outer strength members, thus preventing strain from aerial installation from impairing or otherwise inversely impacting the performance of the optical fibers. Moreover, fibers are loosely housed in bundles to permit fiber movement and further prevent strain on the fibers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/US2005/047177, filed Dec. 23, 2005, the content of which isincorporated herein by reference.

TECHNICAL FIELD

The technical field of this invention is all-dielectric self-supporting(ADSS) cables that contain optical fibers. More specifically, the fieldof this invention relates to ADSS cables that contain more than 288optical fibers.

BACKGROUND

Communication cables that include optical fibers have been deployed inmany types of installations. For example, fiber optic cables are ofteninstalled underground, either by burying them directly or by blowingthem through ducts. Another installation option has been to string thecables aerially between poles, as with traditional telephone lines.

Of these methods, aerial installation has gained popularity. It costsless to deploy cables above ground than below ground, and aerialinstallation makes the fiber optic cable easier to access formaintenance or repair. Moreover, cables installed above ground tend tobe less susceptible to damage, which may happen to cables installed inground by unintentional excavation.

While optical fiber cables are typically installed aerially bysuspending them between poles, this technique applies stresses to thecable that cables in other installations do not face. For instance,aerial installation imparts substantial tensile stresses on the cablecaused by the weight of the cable suspended between poles. Wind, snow,and ice can increase these stresses. Exposure to the environment alsocan subject the cable to thermo stresses from the climate. The tensileand thermo stresses can increase attenuation in the optical fibers,adversely impacting their performance as a communication medium. Lashingthe cable to suspension wires may decrease tensile stresses, but itintroduces other problems. Namely, suspension wires significantlyincrease the cost of installation and, as conductors, may attractlightning. Lightning strikes can seriously damage the fiber optic cable.

In short, fiber optic cables installed aerially need to withstand theincreased stresses that arise from suspension and need to avoidattracting lightning strikes. Conventional cables of this type aretypically of the loose-tube design, where the fibers are housed in aplurality of buffer tubes stranded around a central strength member. Theloose-tube design permits the fibers to move within the buffer tubes andavoid absorbing stress or strain on the cable. Moreover, the materialsin the cable are exclusively dielectric to avoid lightning and allowingthe cable to be placed in the power region of the pole. The cables are,therefore, called all-dielectric, self-supporting (ADSS) cables.

ADSS cables are designed to reduce stresses on the optical fibers. Fiberstrain is a loss mechanism in optical fibers that may occur if the cableis subjected to tensile forces, either from installation or temperature,or compression forces. Fiber strain may cause signal loss in the opticalfibers. A central strength member and usually outer strength members areincluded in ADSS cables to help bear the tensile and thermo stresses.Also, the optical fibers often have excess length so that they may movefreely within the buffer tubes.

FIG. 1 shows a generalized cross-sectional view of a typical ADSS cable102. ADSS cable 102 includes, at its core, a central strength member104, which is capable of withstanding and controlling the significanttensile and thermo stresses that the ADSS cable may be subject to.Typically, central strength member 104 may be made from glass-fiberreinforced plastic. Central strength member 104 may have a jacket orcoating 106 of polymeric material, such as, for example, a polyolefin orpolyethylene coating.

A plurality of buffer tubes 108 surrounds central strength member 104.Each buffer tube 108 includes a plurality of optical fibers 110 withinit. A gel-based filling material may be introduced inside buffer tube108 to serve as a physical barrier to any water accidentally penetratedinside buffer tube 108. A water-swellable tape 112, an inner jacket 114which is used to isolate the optical core, and outer strength members116 respectively surround buffer tubes 108. An outer jacket 118 protectsthe exterior of the cable. A rip cord 120 provides a means for easilyopening the cable jacket to access the fibers during installation orrepair.

Known ADSS cables having the structure of FIG. 1 have had a maximumcapacity of 288 fibers. Conventional ADSS cables with higher fibercounts have followed one of two alternative approaches.

In one design, shown in FIG. 2, a second layer of buffer tubes is addedaround the first layer. In this two-layered design, a first or innerlayer of buffer tubes 202 is directly in contact with or stranded tocentral strength member 204, similar to the design in FIG. 1. Toincrease the fiber count, a second or outer layer of buffer tubes 206 isplaced over and secured to the first or inner layer of buffer tubes 202.The buffer tubes in the second layer have substantially the samedimensions as the tubes in the first layer. A water blocking orswellable tape 208 may be inserted between the two layers 202 and 206.Other features of the two-layered design may be similar to those of theADSS cable of FIG. 1.

In another design, loose fibers in the conventional ADSS cable of FIG. 1are replaced with ribbon fibers. Optical fiber ribbons are planar arraysof fibers that are bonded together as a unit. Through bonding, ribbonsprovide a higher density of fibers per unit area. Ribbons canadvantageously be mass fusion spliced, saving setup and maintenancecosts. Consequently, for the same cable structure, an ADSS cable cangenerally provide a higher number of fibers using ribbons rather thanloose or bundled fibers.

U.S. Pat. No. 6,185,351 describes an ADSS cable using ribbon fibers.FIG. 3 reproduces a cross-sectional view of the cable from the '351patent. As shown in FIG. 3, stacks of ribbon fibers 302 are encased insix buffer tubes 304 in cable 300, leading to a total fiber count inexcess of 288. Depending on the fiber count, the ribbon stacks 302 incable 300 may be rectangular or square in shape. The ribbon stacks 302are generally twisted into a helix to help maintain the stack form.Generally, the optical fibers of the ribbon stacks 302 are held togetherusing an ultraviolet-curable matrix bonding material or other suitableboding material.

Applicants have noted that the known attempts for an ADSS cable having afiber count in excess of 288 have several disadvantages. The two-layerdesign of FIG. 2, for example, exposes the optical fibers to excessivestress in an aerial installation. Specifically, being in immediatecontact with central strength member 204, the first layer of buffertubes 202 is generally well protected from tensile and thermo stressesfrom the environment. In static applications, such as in directly buriedor duct applications, where straining of the cable is minimum, thesecond layer of buffer tubes 206 may also be adequately protected.However, the inner layer of buffer tubes can become decoupled from theouter layer and cause problems either immediately after installation orover time. Moreover, ADSS cables in aerial installations are subject tosignificant Aeolian vibration, direct exposure to hostile environmentalconditions, and other conditions that create substantial tension andstrain on the cable. In such strained conditions, there is less controlover the expansion and/or contraction of the second layer of buffertubes 206.

Additionally, securing a second layer of buffer tubes to an inner layerof buffer tubes causes extra stress and tension to be exerted on theinner layer of buffer tubes. Because an ADSS cable must carry the weightand installation tensions of the cable itself as well as the externalloads created by the effects of wind and ice, the added stress from asecond layer of buffer tubes is undesirable and may cause dataattenuation and other unpredictable irregularities in the fibers in theinner layer of buffer tubes.

The design 300 using optical fiber ribbons also has severaldisadvantages. The fibers located at the corners of the stack may besubject to flexural stresses and may encounter friction from rubbingagainst the inner buffer tube walls. This may result in someunpredictable variations in attenuation in the corner fibers. One way tominimize this unpredictable attenuation of the corner fibers is toselect corner fibers based on mode field diameter and cutoff wavelength.However, such selection is merely a way to minimize the impact of theproblem associated with using ribbon stacks, not really solving theproblem. Another disadvantage of using ribbon stacks is that the rigidshape of the ribbon arrangement minimizes excess fiber length that maybe stored within the buffer tubes. Excess fiber length is desirable inADSS cables. For example, fibers with excess length may move freely whenexposed to environmental stresses and/or when exposed to manipulationssuch as when pulled out of a closure for the preparation of fiber endsfor joining, or for other installation or maintenance relatedactivities. Ribbon designs that have diminished excess fiber length arethus disadvantageous.

ADSS cables with ribbon fibers also suffer from having a comparativelysmall strain-free window. The strain-free window refers to the amount ofaxial load that can be applied to a cable before more than negligibleamounts of strain (>0.1%) are imparted to the optical fibers within thecable.

Generally, cables with ribbon fibers in buffer tubes have smallerstrain-free windows than cables with loose fibers in buffer tubes. Theribbon fibers are more constrained and cannot move as freely to avoidabsorbing the strain placed on the cable.

The '351 patent in its FIG. 3 indicates that a high fiber count ADSScable using ribbon fibers can achieve negligible strain on the opticalfibers at about 0.18% cable strain. Moreover, it states that fiberstrain increases optical attenuation and that the ADSS ribbon cable canachieve negligible attenuation for fiber strain up to approximately0.275%. While the '351 patent discusses “packing density” and“clearance” in buffer tubes to permit fiber movement, Applicants haveobserved that achieving low-fiber strain in an ADSS cable having ribboncables also requires large amounts of aramid fibers as an outer strengthmember system to attain a high enough modulus of elasticity for thecable to protect the fibers from stresses.

Applicants have noticed that the existing approaches for high fibercount ADSS cables do not provide a desirable balance between a largenumber of optical fibers in a single layer self-supporting cable and lowsusceptibility to strain on the optical fibers. Therefore, Applicantshave perceived the need to provide a high fiber count ADSS cable whichdoes not present the drawbacks of high fiber count ADSS cables known inthe art wherein ribbon fibers or, alternatively, at least two layers ofbuffer tubes are used.

SUMMARY

Applicants have found that the drawbacks mentioned above can be avoided,or at least remarkably reduced, by providing a high fiber count ADSScable in which a single layer of bundled buffer tubes is used, thebuffer tubes being designed to contain a high number of optical fibers(e.g. up to 72 optical fibers or even more) without negatively affectingthe overall cable size. Therefore, in accordance with an aspect of thepresent invention, an all-dielectric self-supporting optical fiber cablecomprises a central strength member, a plurality of buffer tubes, atleast 288 bundled optical fibers contained in the buffer tubes, at leastone outer strength member, and an outer jacket. The central strengthmember is longitudinally extending and dielectric. The buffer tubes arelongitudinally extending and helically stranded in a single layer aroundthe central strength member. The optical fibers are loosely arranged inbundles inside the buffer tubes. The at least one outer strength memberis dielectric and extends around the single layer of buffer tubes.Preferably, the all-dielectric self-supporting optical fiber cableaccording to the present invention comprises up to 864 bundled opticalfibers that are contained in the buffer tubes helically stranded in asingle layer around the central strength member. More preferably, theall-dielectric self-supporting optical fiber cable according to thepresent invention comprises up to 432 bundled optical fibers.

In another aspect consistent with the present invention an optical fibercable comprises a resin-based central strength member and a single layerof buffer tubes helically stranded around the central strength member ina reverse-oscillating lay. Each buffer tube contains a water-blockingmaterial and optical fibers grouped into a plurality of bundles. Thesingle layer of buffer tubes contains at least 288 optical fibers.Water-swellable tape surrounds the single layer of buffer tubes, and aninner jacket surrounds the water-swellable tape. Outer strength membersare arranged exterior to the inner jacket, and an outer jacket forms theexterior of the cable. The optical fiber cable consists of dielectricmaterials and is self-supporting in an aerial installation.

With cables consistent with the present invention, the buffer tubes arestranded in an S-Z configuration with a lay length of less than about220 millimeters. Moreover, cable elongation up to 0.55% under axial loadresults in less than 0.1% strain on the optical fibers.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a cross-sectional diagram of a conventional ADSS cable.

FIG. 2 is a cross-sectional diagram of a conventional ADSS cable havingtwo layers of buffer tubes.

FIG. 3 is cross-sectional diagram of a conventional ADSS cable havingfiber ribbon stacks within the buffer tubes.

FIG. 4 is cross-sectional diagram of an ADSS cable having fiber bundleswithin a single layer of buffer tubes in accordance with one aspect ofthe present invention.

FIG. 5 is a graph of cable and fiber elongation under a range of axialloads for the ADSS cable depicted in FIG. 4.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments consistent with theprinciples of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

In accordance with an aspect of the present invention, an all-dielectricself-supporting optical fiber cable comprises a longitudinallyextending, dielectric, central strength member; a plurality oflongitudinally extending buffer tubes helically stranded in a singlelayer around the central strength member; at least 288 optical fibersloosely arranged inside the buffer tubes; an assembly of dielectricouter strength members extending around the single layer of buffertubes; and an outer jacket surrounding the outer strength member.

Generally referenced as 400 in FIG. 4, a cross-sectional diagram of anADSS cable has a dielectric central strength member 402 along its axis.This central strength member 402 possesses substantial tensile andcompressive strength and helps cable 400 resist thermal expansion andcontraction. Preferably, the central strength member 402 comprises glassfiber and resin material 404. As an example, the central strength membermay be made of Glass Reinforced Plastic (GRP). Additionally, centralstrength member 402 may be covered with an extruded polymeric coating406, e.g. made from medium density polyethylene (MDPE). In oneembodiment, central strength member 402 is a 5 mm diameter rod of GlassReinforced Plastic (GRP) which is covered with a MDPE coating up to afinal outer diameter of the central strength member 402 of about 6.25mm.

A plurality of optical assemblies 408 surrounds central strength member402. The optical assemblies each include a buffer tube 410 surroundingand loosely housing optical fiber bundles 412 within them. Preferably,buffer tube 410 contains 6 optical fiber bundles 412. Preferably, eachoptical fiber bundle 412 contains 12 optical fibers. Buffer tubes 410may be made of plastic or other flexible materials substantiallyimpervious to water. A suggested plastic material for buffer tubes 410is a polyolefin selected from the group of polybutylene terephtalate(PBT), polyethylene (PE), polypropylene (PP), or combinations thereof.Preferably, buffer tubes 410 are made of high density polyethylene(HDPE). Preferably, buffer tubes 410 are uniquely distinguishable fromeach other, such as by using a different color for each buffer tube. Forexample, respective buffer tubes 410 may be colored blue, orange, green,brown, slate, and white, respectively, following industry customs.

Buffer tubes 410 preferably are extruded around a group of fiber bundles412 in a known manner. The size of the buffer tubes may vary with thefiber capacity, and any suitable size of buffer tube may be used withinthe scope of the present invention. As an example, each buffer tube 410has an outer diameter of 6.2 mm and an interior diameter of 4.8 mm. Thenumber of buffer tubes in the single layer may also vary with fibercapacity. In. the particular example of FIG. 4, six longitudinallyextended optical assemblies 408 surround central strength member 402.

Buffer tubes 410 are generally wrapped around central strength member402 in a reverse helix or “S-Z” fashion. This stranding is also known asa reverse oscillating lay. The locations at which the stranded tubesreverse direction (e.g. from an “S” to a “Z”) are referred to asreversal points. S-Z stranding of buffer tubes in general, and thereversal points in particular, facilitate accessing the optical fiberswithin the middle of the cable span and to allow branching of the cableto other optical paths. The S-Z stranding provides sufficient excess oftube length to make the tap easy by opening the side of the cable at apoint along its length without losing the desired slack in the opticalfibers within the tube that is opened. As one example, buffer tubes 410are stranded over central strength member 402 using a 200 mm bend radiuswith a 220 mm lay length (preferably less than about 220 mm). A binderthread or threads may be contrahelically applied around buffer tubes tohold them in place.

Each optical assembly 408 in FIG. 4 also includes fiber bundles 412within buffer tubes 410. Optical fibers within bundle 412 are any typeof optical fiber waveguide known by those skilled in the art. A binderthread or tape (not shown) or similar device serves to separably holdthe discrete optical fibers in close proximity. The binder thread may behelically applied to fiber bundle 412 with, for example, 100 mm laylength. The binder thread may be color-coded to distinguish one bundle412 from other bundles in the same buffer tube, in a manner well knownin the art. Alternatively, a colored plastic jacket (not shown) can beused to encase bundle 412. The jacket may be any plastic material andwould preferably be made of polyvinyl chloride (PVC).

The number of fiber bundles 412 and the number of fibers within a bundlemay vary depending on the particular application. FIG. 4, as an example,depicts each buffer tube 410 with six bundles of 12 fibers, giving eachbuffer tube a total of 72 optical fibers. With six buffer tubes, cable400 has 432 fibers.

Water blocking material 414 may be inserted inside buffer tube 410 andaround fiber bundles 412 to prevent water ingress and damage. Forinstance, the tubes may be flooded with a conventional thixotropic gel.The gel not only protects the fibers from water but also supports fiberbundles 412 within buffer tubes 410 to help insulate them from stressesimparted on the cable.

A conventional water-swellable tape 416 may be wrapped around thecollection of optical assemblies 408 and extended longitudinally alongthe entire length of ADSS cable 400. For example, a 2.75 inch (about 70mm) wide water-swellable tape may be applied over the single layer ofbuffer tubes 410, as shown in FIG. 4, and bound to them using, forexample, a polypropylene binder. The tape may be, for example, apolymeric based tape that has on its surface a superabsorbent swellablematerial.

An inner jacket 418 may surround water-swellable tape 416. Inner jacket418 may be formed by extruding a polymeric material around thewater-swellable tape 416.

At least one outer strength member 420 is placed over inner jacket 418.Preferably, an assembly of outer strength member 420 is placed overinner jacket 418. Outer strength members 420 may include multiplestrands of material having high tensile strength. The members 420,together with central strength member 402, help to increase the modulusof elasticity of the overall cable 400 and minimize strain on theoptical fibers within buffer tubes 410. Typically, strength members 420are aramid strands or glass threads. A first half of the strands may bewrapped in a clockwise direction around the inner jacket 418. The secondhalf of the strands in the outer strength member assembly 420 may bewrapped in a counter-clockwise direction around the inner jacket. As anexample, the outer strength member assembly 420 includes 25 ends of 8050Dtex aramid yams.

A barrier tape 422 may be applied on top of the outer strength memberassembly 420. Barrier tape 422 may be a water-swellable tape. Forexample, a 3.25 inch (about 83 mm) wide water-swellable tape may beused. Tape 422 provides additional protection for the optical fibersfrom potential water ingress and migration in the cable 400. A polyesterbinder thread or other mechanism may help retain the water-swellabletape 422 against the cable.

An outer jacket 424 forms the exterior of cable 400. Jacket 424 may beformed by extruding a polymeric material around barrier tape 422. A pairof rip cords 426 may be applied beneath the outer jacket 424 to provideaccess to internal compounds of the ADSS cable, for example, during afield application.

As illustrated by ADSS cable 400 in FIG. 4, high fiber count may beachieved in an ADSS cable with only a single layer of buffer tubes. Inthis example, if each buffer tube 410 holds six 12-fiber bundles, theillustrated ADSS cable 400 will have a total of 432 optical fibers.

Applicants have constructed cable 400 and verified that its performancepasses the test requirements of Telcordia GR-20, Issue 2 and IEEE Std1222-2004. Those tests include temperature cycling, cable aging, cablecyclic flexing, cable twist, compressive strength, impact resistance,water penetration, sheave, and stress/strain.

Unlike a high-fiber count ADSS cable using two layers of buffer tubes asin FIG. 2, the high-fiber count cable of FIG. 4 provides aself-supporting design that avoids the potential for crushing of opticalfibers in an internal layer of buffer tubes. The cable consistent withthe present invention also avoids the potential for decoupling of thebuffer tube layers after installation due to high frequency vibrations.

Unlike a high-fiber count ADSS cable using ribbon fibers as in FIG. 3,the high-fiber count cable of FIG. 4 provides a large strain-free windowto minimize the risk of signal attenuation in a self-supportinginstallation. FIG. 5 is a graph showing the amount of elongation for thecable and fibers, respectively, of FIG. 4 for various axial loads. Asshown in FIG. 5, cable 400 has a strain-free window of about 0.55-0.60%,i.e., the cable can elongate up to about 0.55-0.60% (at nearly 3200 lbs)before the optical fibers stretch 0.1%. FIG. 3 of the '351 patent showsthat the strain-free window for an ADSS cable having ribbon fibers isonly about 0.18-0.2%, i.e. the cable can elongate up to about 0.18-0.2%(at 600 lbs) before the optical fibers stretch 0.1%. The cable of FIG. 4achieves the low strain performance without having to include excessiveamounts of aramid strength members. For example, using the cable of FIG.4 as an example, it would take approximately one hundred 8050 Dtexaramid yams to limit the cable elongation to 0.2% at an applied axialload of 3200 lb. This is an increase of approximately 75 yarns over thenumber that, according to the present invention, is found to besufficient to provide strain free operation of the FIG. 4 cable usingbundled fibers.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

1. An all-dielectric self-supporting optical fiber cable, comprising: alongitudinally extending, dielectric, central strength member; aplurality of longitudinally extending buffer tubes helically stranded ina single layer around the central strength member with a lay length nothigher than 220 mm; more than 288 optical fibers bound together asbundles with color-coded thread inside the single layer of buffer tubes;an inner polymeric jacket extending around the single layer of buffertubes; at least one dielectric outer strength member positioned aroundthe inner jacket and, together with the central strength member,increasing the modulus of elasticity of the all-dielectricself-supporting optical fiber cable, the outer strength member includingmultiple strands of material having high tensile strength, wherein afirst half of the strands is wrapped in a clockwise direction around theinner jacket and the second half of the strands is wrapped in acounter-clockwise direction around the inner jacket; and an outer jacketsurrounding the outer strength member, wherein the optical fiber cableis self-supporting in an aerial installation.
 2. The all-dielectricself-supporting cable of claim 1, wherein elongation of the cable of upto 0.55% under axial load results in less than 0.1% strain on theoptical fibers.
 3. The all-dielectric self-supporting cable of claim 1,wherein the buffer tubes are stranded in an S-Z configuration.
 4. Theall-dielectric self-supporting cable of claim 1, wherein at least 432optical fibers are bundled inside the buffer tubes.
 5. Theall-dielectric self-supporting cable of claim 4, wherein at least 864optical fibers are bundled inside the buffer tubes.
 6. Theall-dielectric self-supporting cable of claim 1, further comprising awater-swellable tape positioned between the at least one outer strengthmember and the outer jacket.
 7. The all-dielectric self-supporting cableof claim 1, further comprising water-blocking material within theplurality of buffer tubes.
 8. An optical fiber cable, comprising: acentral strength member; a single layer of buffer tubes helicallystranded around the central strength member in a reverse-oscillating laywith a lay length not higher than 220 mm, each buffer tube containingoptical fibers grouped into a plurality of bundles with color-codedthread, the single layer of buffer tubes containing more than 288optical fibers; an inner polymeric jacket extending around the singlelayer of buffer tubes; at least one outer strength member positionedaround the inner jacket and, together with the central strength member,increasing the modulus of elasticity of the optical fiber cable, theouter strength member including multiple strands of material having hightensile strength, wherein a first half of the strands is wrapped in aclockwise direction around the inner jacket and the second half of thestrands is wrapped in a counter-clockwise direction around the innerjacket; and an outer jacket in a radial outer position with respect tosaid at least one outer strength member, wherein the optical fiber cableconsists of dielectric materials and is self-supporting in an aerialinstallation.
 9. The cable of claim 8, wherein at least 432 opticalfibers are bundled inside the buffer tubes.
 10. The cable of claim 9,wherein at least 864 optical fibers are bundled inside the buffer tubes.11. The cable of claim 8, wherein elongation of the cable of up to 0.55%under axial load results in less than 0.1% strain on the optical fibers.12. The cable of claim 8, wherein the buffer tubes are stranded in anS-Z configuration.
 13. The cable of claim 8, wherein the centralstrength member is made of a resin material.
 14. The cable of claim 8,wherein each buffer tube contains a water-blocking material.
 15. Thecable of claim 8, further comprising a water-swellable tape surroundingthe single layer of buffer tubes.